The use of a test device in which a carrier matrix, such as paper, is impregnated with an indicator for glucose is described in U.S. Pat. No. 2,981,606. Such a device is said to be useful in determining glucose in blood.
The '606 patent teaches a glucose indicator comprising a mixture of glucose oxidase, peroxidase and a color-forming substance such a benzidine or o-tolidine. By impregnating paper with such a composition and drying, a test paper is obtained which can be used to measure glucose by contacting it with a drop of blood or urine and observing any color formed.
But the device of the '606 patent has a significant drawback where blood is the intended test sample: the colored matter in blood interferes with easy observation of the color produced by a positive test. This problem was addressed by the patentees of U.S. Pat. Nos. 3,092,465 and 3,298,789. Both of these patents deal with various aspects of a test device for testing for glucose in blood. Both describe test devices having an enzyme system in a porous carrier matrix such as in the '606 patent.
This impregnated composite is protected from the staining propensity of hemoglobin and other blood components through use of a protective coating, or upper layer, of a membrane permeable to water and glucose, but which screens out relatively large molecules. Thus, interfering pigments such as hemoglobin can be washed off prior to observing the device for the appearance of, or change in, color attributable to the presence of glucose. The protective membrane can be cast as a liquid film, for example a solution of cellulose acetate in benzene, which is allowed to dry thereby leaving a continuous, porous film over the reagent-impregnated paper layer.
Similar technology is disclosed in U.S. Pat. No. 3,992,158. This disclosure is directed broadly to an integral test element having at least two layers and comprising a spreading layer and a reagent layer. A liquid sample is contacted with the spreading layer, which is porous and which causes the test sample to spread out over the surface of the spreading layer.
To summarize the background of technological developments leading up to the present invention, devices are known which detect the presence of many analytes present in liquid test samples. For example, glucose in blood can be detected using a paper strip impregnated with glucose oxidase, peroxidase and an appropriate indicator.
Such a device, while having enjoyed marked commercial success in glucose urinalysis, nevertheless has a serious drawback in blood analysis: interference with observing color formation attributable to such blood components as hemoglobin. The '465, '789 and '158 patents addressed this problem by providing a porous film overlay to filter out the interfering blood components, while permitting the aqueous portion of the sample to permeate the film, and to contact the reagent layer.
Such devices permit removal of the interfering components by washing in a stream of water. However, a wipe-off technique has for all practical purposes eluded prior experimenters. The present invention affords the user the convenience of merely wiping off excess sample from the test device thereby eliminating the necessity of washing.